This invention is directed to papermaking processes and systems. More particularly, this invention is directed to adjustment of electrical properties of papermaking compositions.
Paper is made by mixing a number of colloidal, polymeric, and solution components and then allowing the colloidal suspension to flow through a narrow slit onto wire gauze. The paper pulp is a pseudoplastic material with a well-defined yield value. The magnitude of the yield stress and the way in which the viscosity changes with shear rate are important in producing a smooth outflow of the pulp and an appropriate thickness on the moving wire gauze. Those flow characteristics should be monitored and adjusted if necessary.
The colloid science covers a wide range of seemingly very different systems. Many natural and man-made products and processes can be characterized as being colloidal systems. For example, commercial products such as shaving cream and paints, foods and beverages such as mayonnaise and beer, and natural systems such as agriculture soils and biological cells are all colloidal systems.
Colloids in simple terms are an intimate mixture of two substances. The dispersed or colloidal phase in a finely divided state is uniformly distributed through the second substance called the dispersion or dispersing medium. The dispersed phase can be a gas, liquid or solid. The size of colloidal substance present in dispersing medium can vary in size approximately between 10 to 10,000 angstroms (1 to 1000 nanometers)(The American Heritage Dictionary, fourth edition, Houghton Miflin Company, p.365, 2000). The distribution of electric charge and electrostatic potential in the immediate neighborhood of the surface of a colloidal particle is important. The reason for this is that many transport properties, such as electrical conductivity, diffusion coefficient and the flow of many systems are determined by charge distribution.
As indicated above, a papermaking composition (or paper furnish) is generally made up of materials (fiber, filler, etc.) and a bulk phase, normally water, containing dissolved and colloidally dispersed materials (salts, polymers, dispersants, etc.). Although the overall, or average charge of the total furnish (particulate and water phases) must be neutral (principle of electro-neutrality). However, individual components can be positive (cationic), negative (anionic), or neutral. Morerover, each particle will have a specific average charge, derived from many individual cationic and anionic sites, and the water phase will have an “average” charge from dissolved and colloidal matter.
The surface chemical properties of the fibers and fines depend on chemical composition of the surface of the fiber or fine. For example, pulp fibers resulting from mechanical and/or chemical pulping processes, when dispersed in water, acquire a certain charge. There are several ionizable groups that are present in wood pulp, such as hemicellulose and lignin carboxyl groups, lignin phenolic OH groups, sugar alcohol groups, hemiacetal groups, and lignosulphonate groups.
Fiber and fines can also acquire charge, depending upon type and concentration of dissolved substances in the water. For example, dissolved salts tend to have an ion-exchange behavior and resulting charge on pulp fibers can either be negative (or) positive (or) neutral. The strength of attraction (ion adsorption) by the carboxyl groups is a function of ion valency and species. The strength of attraction of wood fibers for various ions occurs in the following order: Na+<K+<Ag+<Ca2+=Mg2+=Ba2+<Al3+ (William E.Scott, Wet End Chemistry, TAPPI, Ed.1996, page 16.).
Additives ae equally important with respect to the above issues. Many of the additives listed in Table 1 have a surface charge. The type and intensity of charge vary based on the additive used. These chemical include retention aids, flocculants, drainage aids, resins, dispersants, chelants, scale inhibitors, corrosion inhibitors, slimicides, and the like.
TABLE 1Wet End Chemical AdditivesInternal sizesCationic flocculantsExternal sizesAlum (papermakers alum), andRosins(colophony), typicallyalum substitutes such asfatty organic acids, such aspolyaluminimum chloride,abietic acidpolyaluminium hydroxychloride,Rosin soaps (for exampleand polyaluminium silicate sulfatesodium abietate)DyesStarch sizesAcid dyes, typically used with aCereal starch (corn, wheat)dye fixing agentTuber starch (for exampleBasic dyespotato, tapioca)Direct dyesUnmodified starchesPigment dispersionsModified starchesLiquid sulfur dyesOxidized starchesOptical brightening agents (OBA)Starch (cationic/anionic)Diaminostilbene disulfonic acidAmphoteric starchesderivativesStarch estersOBA quenchersHydrophobic starchesQuaternary polyamidesAcid modifided starchesRetention aids, drainage aidsHydrolyzed starchesSingle polymer systemsAlklaine (neutral) sizesPolyacrylamidesAlkyl ketene dimmer (AKD)PolydiallyldimethylammoniumAlykenyl succinic anydridedchloride(ASA)PolyethyleneimineNeutral rosin sizesAcrylic acid/acrylamide polymersWax(either paraffin orDual polymer systemsmicrocrystalline)FluorochemicalsDry strength resins (such asstyrene-acrylate copolymers,styrene-maleic anydridecopolymers, polyacrylamides,polyurethane, and polyvinylalcohols)
The type of water used, and variations in process conditions employed, can also influence the amount and quantity of ions present. The current industrial trend is to minimize the use of fresh water during papermaking and recycle more and more of the process water. Recycling the process water increases ions built up in the system. The dissolved charges in water are mainly due to the presence of various soluble salts present in their ionic form, such as sodium, calcium, chloride and sulfates.
A common method of evaluating surface charge is by determining the zeta potential (rather tham measuring the actual surface charge). Zeta potential is explained as the charge potential at the interface plane between the Stern Layer and Gouy-Chapman region of an electrical double layer. The strength of these potentials and the distance involved determine the resistance of hydrophobic suspensions to coagulate or flocculate (William E.Scott, Wet End Chemistry, TAPPI, Ed.1992, page 3-4). Zeta potential is frequently used by papermakers as an indication of the state of electrokinetic charge in the system.
The use and measurement of zeta potential offers several benefits to a papermaker. It can provide adsorbing capacity of pulp fibers to a given additive. It can also help to choose the type of additive required to achieve a charge balance. Moreover, it can be used to predict upsets by flagging deviations from a set point.
Some representative disclosures of zeta measurement and its advantages to papermakers include: WO 99/54741 A1 (Goss et al.), EP 0 079 726 A1 (Evans et al.), WO 98/12551 (Tijero Miguel), and U.S. Pat. No. 4,535,285 (Evans et al.), “Wet-End Chemistry of Retention, Drainage, and Formation Aids”, Pulp and Paper Manufacture, Vol. 6: Stock Preparation (Hagemeyer, R. W., Manson, D. W., and Kocurek, M. J., ed.), Unbehend, J. E., Chap. 7: 112-157 (1992), “Use of Potentiometric Titration and Polyelectrolyte Titration to Measure the Surface Charge of Cellulose Fiber”, Gill, R. I. S., Fundamentals Pmkg. (Baker & Punton, ed.) Trans. 9th Fundamental Res. Symp. (Cambridge), Vol. 1: 437-452 (September 1989), “Adsorption of Ions at the Cellulose/Aqueous Electrolyte Interface”, Harrington, T. M.; Midmore, B. R, JCS Faraday I 80, no. 6: 1525-1566 (June 1984), “SURFACE PHENOMENA”, Clark, J. d'A, Pulp Technol. & Trmt. for Paper (Miller Freeman Publns.), Chap. 4: 87-105 (1978), “ADSORPTION AND FLOCCULATION MECHANISMS IN PAPER STOCK SYSTEMS”, Britt, K. W.; Dillon, A. G.; Evans, L. A., TAPPI Papermakers Conf. (Chicago) Paper IIA-3: 39-42 (Apr. 18-20, 1977), and ZETA-POTENTIAL MEASUREMENTS IN PAPER MANUFACTURE”, Lindstrom, T.; Soremark, C., Papier 29, no. 12: 519-525 (December 1975).
The zeta potential values measured during papermaking are system dependent and change due to process variations and upsets. Considerable deviations in zeta potential from a system's optimum will affect the production and quality of cellulose products. Generally speaking, many have proposed that a zeta close to zero or slightly negative is desirable. However, a targeted zeta potential value for a specific paper machine is a function of several factors, such furnish type, production rates, product grades, the ambient conditions, the particular operator on duty, the particular starting materials, and additives.
One way of avoiding or rectifying zeta deviations or flagged upsets is by adjusting the papermaking process by introducing additives to various portions/stages thereof. However, introduction of additives has significant drawbacks.
First, introducing additives to the process presents unknown chemical interactions with the papermaking composition. Unforeseen chemical reactions may result in reaction products whose effect upon the process is undesirable. Without more knowledge of these chemical reactions, it is difficult to adjust the process conditions to rectify the undesirable effect.
Secondly, introducing additives to the process over time creates a buildup of the additives and of the known reaction products of the additives and components of the papermaking composition. Once an upper limit of concentration(s) for any or more of these is reached, the process must be shut down. In that case, the operator may be forced to discard pulp or treat it so that it may be recycled. The operator may also have to drain the process of the aqueous components of the papermaking compositions, and replenish them with fresh water and additives. Most importantly, production is significantly decreased.
Thirdly, introducing additives to the process also complicates the physical interactions of fibers, colloidal species and dissolved species within the papermaking composition. For example, if colloids having a significant surface charge are not suitably neutralized, they may agglomerate with oppositely charged species, thereby resulting in flocculation at an inappropriate time during the process. Conversely, agglomeration and flocculation may not occur at the appropriate time, or at all, if the colloids do not have a sufficient charge, i.e., they remain suspended in the aqueous phase.
Fourthly, some additives may undesirably react with various mechanical parts in the process. Corrosion of these parts over time may lead to mechanical breakdowns. As a result, the process must be shut down and the part at issue repaired or replaced. This is often very costly.
Despite the above drawbacks, many have proposed addition of cationic or anionic chemical additives. Several have proposed various strategies for this type of modification.
U.S. Pat. No. 6,072,309 (Watson et al.) suggests the use of electrolytes such as cations (including dissolved aluminum and iron cations) in order to adjust the zeta potential.
U.S. Pat. No. 5,365,775 (Penniman) discloses adjustment of the zeta potential via addition to the papemaking process of an appropriate polymer.
The abstract from “INTERFACIAL PROPERTIES OF POLYELECTROLYTE-CELLULOSE SYSTEMS; ELECTROKINETIC PROPERTIES OF CELLULOSE FIBERS WITH ADSORBED MONOLAYERS OF CATIONIC POLYELECTROLYTE”, Onabe, F., J. Appl. Polymer Sci. 23, no. 10: 2909-2922 (May 15, 1979) discloses zeta-potential measurements on acetate-grade dissolving pulp fibers with and without irreversibly adsorbed monolayers of cationic polyelectrolyte, viz., poly(dimethyl diallyl ammonium chloride). As the amount of adsorbed polymers increased, the negative zeta-potential of the fibers decreased until the polarity of the zeta-potential was reversed to the positive side. A marked change in the value of zeta-potential was not observed when the formation of the saturated monolayer was completed. The abstract suggests that the charge of the cellulose fibers can be controlled until formation of a saturated monolayer of cationic polyelectrolytes if the number of adsorbed segments per unit area of fiber surface at saturated monolayer formation is greater than the number of carboxyl groups per unit area of fiber surface
The abstract for “COMPARATIVE EVALUATION OF ELECTROKINETIC BEHAVIOR OF POLYELECTROLYTE-CELLULOSE SYSTEMS”, Onabe, F., J. Soc. Fiber Sci. Technol. Japan (Sen-i Gakkaishi) 34, no. 11: T494-504 (November 1978) discloses studies conducted to elucidate the mechanism of electrostatic charge control in pulp fibers by cationic wet-end additives and the function of counterions in controlling the surface electric charge. In systems with irreversibly adsorbed polymer layers, the negative zeta-potential of fibers with monolayers reversed polarity to a positive value, whereas the zeta-potential for multilayers remained negative with increased salt concentrations. Among systems containing counterions of various valencies, the polarity of both positively and negatively charged fibers reversed upon increase of salt concentration. Of the two systems simulating paper-machine wet-end operation, negatively charged fibers remained negative with increased alum additions, but reverted to a positive charge upon increased dosage of the polyelectrolyte. Electric double-layer models are proposed to account for the electrokinetic behavior of the systems. The significance of specific adsorption of polyvalent counterions for effective charge control on the fibers is demonstrated.
The abstract for “DRAINAGE AND RETENTION MECHANISMS OF PAPERMAKING SYSTEMS TREATED WITH CATIONIC POLYMERS”, Moore, E. E., Tappi 58, no. 1: 99-101 (January 1975) discloses that optimum drainage or retention of a papermaking system in which a drainage and retention aid is used does not necessarily correlate with the point of charge neutralization of the substrate surface. In a bleached pulp suspension containing alum, drainage or retention can increase greatly with increasing amounts of cationic polyacrylamide, even though the fiber surface has been charge reversed. The lack of correlation of these props. with zero zeta-potential shows that mechanisms other than charge neutralization may predominate.
The abstract for “IMPORTANCE OF ELECTROKINETIC PROPERTIES OF WOOD FIBER FOR PAPERMAKING”, Lindstrom, T.; Soremark, C.; Heinegard, C.; Martin-Lof, S., Conference: TAPPI Papermakers Conf. (Boston), TAPPI Papermakers Conf. (Boston): 77-84 (Jun. 3-6, 1974) discloses varing of the zeta potential and thus the tendency for flocculation by adding cationic polyacrylamides (PAA) to dispersions of cellulosic matl. (microcryst. cellulose sol). Optimum flocculation occurred at a zeta potential of ca. zero. Mill trials to determine a correlation between zeta potential and single pass retention on the wire showed increased retention as the zeta potential was lowered.
The abstract for “RETENTION AND RETENTION AIDS”, Ninck Blok, C. J. J.; Klein, B. de, Papierwereld 22, no. 3: 69-81 (March, 1967) discloses a clear relation of cationic retention aids adsorption to exposed fiber surface. Zeta-potential measurements of pulp fibers as a function of adsorbed amount of cationic retention aids show a change from negative to positive charge values. It suggests that increased retention is probably due to changes in zeta-potential.
The abstract for “Online Cationic-Demand Measurement for Wet-End Papermaking”, Veal, C., 1997 Engineering & Papermakers: Forming Bonds for Better Papermaking Conference, (TAPPI Press): 287-296 (Oct. 6, 1997; TAPPI Press) discloses optimized control of cationic materials enhances strength properties and improves runnability, drainage, and formation through measurement of colloidal and dissolved charge demand to determine or detect changes in furnish charge characteristics before the stock reaches the paper machine.
The abstract from “Starches for Surface Sizing and Wet-End Addition”, Brouwer, P. H., Wochenbl. Papierfabr. 124, no. 1: 19-23 (Jan. 15, 1996) discloses that paper-machine wet-end operation gives the best results when electric charges at both the fiber surface (zeta potential) and in the aqueous phase (soluble charge) are near zero, and suggests that suitable cationic additives (such as polyacrylamide) be used.
Still others have proposed addition of other additives.
The abstract from “Interactions Between Cationic Starches and Papermaking Fibers; Effect of Starch Characteristics on Fiber Surface Charge and Starch Retention”, Gupta, B. Scott, W., 1995 Papermakers Conference: Proceedings (TAPPI): 85-96 (Apr. 26, 1995; TAPPI Press) discloses that, in terms of time-dependent behavior, starch DS and dosage level were the most significant factors affecting surface charge, and suggests that, when selecting a starch for a particular application, starch-retention measurements should be carried out and that starch DS and dosage levels should be the variables manipulated.
The abstract for “INFLUENCE OF ALUM AND pH ON THE ZETA POTENTIAL OF FIBERS AND ADDITIVES”, McKenzie, A. W.; Balodis, V.; Milgrom, A., Appita 23, no. 1: 40-4 (July, 1969) discloses that the negative charge normally found on fibers, on starch, and on titanium dioxide could be reversed in the presence of the Al sulfate. In most cases, the reversal of charge resulted from the adsorption of colloidal alumina on the surface of the fiber or the additives.
Outside of the above area of electrical properties, some have proposed adding carbon dioxide (CO2) to papermaking processes for a variety of reasons.
WO 99/24661 A1 discloses improvement of drainage of a pulp suspension by treating it with carbon dioxide just before a dewatering device.
U.S. Pat. No. 2002/0092636 A1 and U.S. Pat. No. 6,599,390 B2 disclose addition of carbon dioxide in several reactors containing pulps including calcium hydroxide or calcium oxide in order to precipitate different forms of calcium carbonate.
U.S. Pat. No. 2002/0148581 A1 discloses regulation of broke pH with addition of carbon dioxide.
U.S. Pat. No. 2002/0162638 A1 discloses precipitation of additives in pulp suspensions with carbon dioxide having lowered purity.
U.S. Pat. No. 2002/0134519 A1 discloses eliminating detrimental substances by forming metal hydroxides through pH control with carbon dioxide.
U.S. Pat. No. 6,251,356 B1 discloses precipitation of calcium carbonate from a pressurized reactor containing calcium hydroxide or calcium oxide.
U.S. Pat. No. 6,436,232 B1 and U.S. Pat. No. 6,537,425 B2 disclose addition of carbon dioxide to pulps containing calcium hydroxide in order to precipitate calcium carbonate.
Despite these disclosures, none have recognized interaction between carbon dioxide and electrical properties of the papermaking composition, such as zeta potential, conductivity and electrical charge demand. None of them have disclosed addition of carbon dioxide to papermaking compositions based upon measurement of electrical properties of a papermaking composition, such as zeta potential, conductivity and electrical charge demand. None have appreciated the advantages of adding carbon dioxide upon the electrical properties of papermaking compositions.
Thus, those skilled in the art will appreciate that there is a need for more suitable additives for papermaking systems in order to adjust electrical properties of papermaking compositions such as zeta potential, conductivity, electrical charge demand, and streaming potential. They will also appreciate that there is a need for an additive that will not tend to build up over time such that the papermaking process must be shut down undesirably frequently. They will further appreciate that there is a need for an additive that will not adversely affect the mechanical parts of a papermaking machine. They will still further appreciate that there is a need for an additive that will improve properties of pulp fiber slurries, diluted pulp fiber slurries, broke, whitewater, paper webs and paper sheets when added to papermaking processes.